This application claims the benefit of European Application No. 00401992.3, filed Jul. 11, 2000.
1. Field of the Invention
The present invention relates generally to optical devices for optical communications, and particularly to a semiconductor optical amplifier (SOA) for use in optical communications.
2. Technical Background
Semiconductor optical amplifiers (SOAs) are optical devices of a structure essentially analogous to that of lasers and which are biased by an injection current or amplifying current (Iamp) below the threshold gain value (gth=g(Iampth)) when the amplifier gain or optical gain reaches the gain threshold:
gth=xcex1xe2x88x92(1/L)lln(r1xc2x7r2)=g(Iampth)xe2x80x83xe2x80x83(1) 
where xcex1 (alpha) equals the losses into the cavity or cavity losses, L is the cavity or device length and r1 and r2 are the facet reflectivities.
Hence, at a given or fixed wavelength, lasing appears as soon as losses are compensated by the material gain as equation 1 suggests. This gain threshold must not be reached to avoid starting of laser oscillations or the lasing effect. However, the SOA is biased above transparency, to exploit the amplification characteristics of the active material in the SOA.
Clamped Gain-SOAs (CG-SOAs) are a type of semiconductor optical amplifiers that have been developed for their benefit of improved input power dynamic range. This attribute enables the CG-SOA to be used as a fast switching optical gate in wavelength division multiplexing (WDM) systems because of the fast electrical control, optical bandwidth, and low intermodulation levels provided by the CG-SOAs. The CG-SOA is made based on the integration or hybridization of an active amplifying section together with one or two passive Bragg reflectors to make the reflectivities r1 and/or r2 sensitive to the wavelength. When laser oscillation begins, the gain of the CG-SOA is clamped or fixed to a given value Gc with respect to the bias current. As a consequence, for a given bias beyond the laser threshold, gain compression arising with high input level does not appear until the amplifier available gain is higher than Gc. This provides a flat optical response over a large input power dynamic range or a wide range of input signal powers (at wavelengths other than the Bragg wavelength). The Bragg wavelength is forced out-of-band or out of the 3 dB optical bandwidth for the CG-SOA and the Bragg grating optical coupling efficiency for low reflectivity must be low in order to make the lasing operation only appear at a high injected current (in order to clamp the gain at a high level).
Basically for a Bragg grating, there is only one wavelength which is fixed for which a non-zero reflectivity value is realized. The Bragg wavelength for a Bragg reflector is given by:
xcexB=2neffxcex9xe2x80x83xe2x80x83(2) 
where xcex9 or xcexA is the spatial period of the grating and neff is the effective optical index of the guiding structure.
When the reflectivity is non-zero, there is a possibility to have a lasing operation at lambdaB or xcexB, the Bragg wavelength, as soon as the amplifier gain is equal to the threshold gain. The gain increases with injected current, so a lasing operation occurs for a given injected current. Now if the injected current is still increased, the lasing effect increases while the ASE spectrum for different wavelengths of lambdaB does not change. This means that for a wavelength different than lambdaB, the amplifier gain does not change if the injected current was increased. This is precisely the property of a Gain Clamped component, and this is more specifically, the well-known principle for the CG-SOA.
However, for some applications, the gain clamped to a given value can be a drawback. Once the optical gain is thus fixed, it can no longer be adjusted as would be desirable for use as a variable optical amplifier (VOA). If multiple CG-SOAs were to be used in an SOA array, it would be desirable to be able to easily vary the gain of an individual SOA in order to equalize the gain of the overall array. Therefore, there is a need for a tunable CG-SOA.
One aspect of the present invention is the teaching of a gain-tunable semiconductor optical amplifier comprising an amplifying section for amplifying an optical signal. A first tunable reflector section and a second tunable reflector section are integrated on opposed sides of the amplifying section to reflect the optical signal at a clamping wavelength.
In another aspect, the present invention includes a distributed Bragg reflector serving as the tunable reflector section.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.